In a wavelength division multiplexing (WDM) optical transmission system, optical signals at a plurality of wavelengths are encoded with digital streams of information. These encoded optical signals, or “wavelength channels”, are combined together and transmitted through a series of transmission links, each link including a span of an optical fiber. At a receiver end of the WDM optical transmission system, the wavelength channels are separated, whereby each wavelength channel can be detected by an optical receiver.
While propagating through an optical fiber, light tends to lose power. This power loss is well understood and is related to the physics of propagation of light in the fiber. Yet some minimal level of wavelength channel power is required at the receiver end to decode information that has been encoded in a wavelength channel at the transmitter end. To boost optical signals propagating in an optical fiber, optical amplifiers can be deployed at multiple locations along a WDM optical transmission system. Optical amplifiers can extend a total length of a WDM optical transmission system to thousands of kilometers, by amplifying optical signals to power levels close to the original levels of optical power at the transmitter end.
There are two main types of optical amplifiers used in fiberoptic transmission systems. The first type is an erbium doped fiber amplifier (EDFA), which uses the phenomenon of stimulated optical emission to amplify light. The second type is a Raman amplifier, which uses the phenomenon of stimulated Raman scattering in the transmission optical fiber to amplify light propagating in the transmission fiber.
There are two types of noise that fundamentally leads to the transmission penalties: 1) noise generated by amplifiers (ASE noise) and 2) transmission fiber induced non-linarites that generate signal which can be described as “nonlinear noise”. In large capacity transmission systems employing coherent multi-level signal formats, the nonlinear noise is proportional to cube of the optical power density. While the former, ASE noise impact increases when optical signal power is small, the later, nonlinear noise impact is larger when the optical signal is large. It is traditional to express ASE noise impact through Noise Figure (NF) of the amplifier. Large values of NF correspond to higher ASE impact on transmission system.
Therefore, it would be significantly more beneficial to have a constant optical power of the signal along the transmission fiber, rather than spatially varying power, since the nonlinearity will be very strong at peaks of the optical power variation along the fiber, and ASE noise impact will be stronger where the signal optical power is low.
Raman amplifiers can have a lower noise figure than EDFA, because they can provide amplification distributed over long length of optical fiber, thus avoiding locations where optical power density is too low. However, Raman gain is normally not distributed evenly along the transmission fiber. This happens because the Raman pump optical power level decays away from the pump source, causing a variation of optical power levels of the optical signal being amplified.
Grubb et al. in U.S. Pat. No. 6,344,922 disclose an optical transmission system including a plurality of Raman pumps. To even out Raman pump light distribution in the transmission fiber, a plurality of fiber Bragg gratings (FBGs) are disposed along the transmission fiber. The FBGs are constructed not to reflect optical signal, while selectively reflecting light of at least some of the Raman pumps. While evening out Raman pump optical power distribution in the transmission fiber, thus improving amplifier NF, FBGs can cause an undesired lasing, destabilizing the transmission system.
Ania-Castañón in an article “Quasi-lossless transmission using second-order Raman amplification and fibre Bragg gratings”, Optics Express 2004 Vol. 12, No. 19, p. 4377, discloses a transmission system including a span of optical transmission fiber pumped by primary pumps disposed at both ends of the transmission fiber. Two fiber Bragg grating (FBG) reflectors are coupled at both ends of the fiber. The central wavelength of the FBG reflectors is 1455 nm, which is close to the Stokes peak of the primary pumps. The pair of FBG reflectors creates a cavity for the radiation at this wavelength. If the primary pumps power is above the threshold necessary to overcome the attenuation of the first Stokes light, a stable secondary pump at 1455 nm is generated in the cavity from the amplified spontaneous emission (ASE) noise at this wavelength. This secondary pump is used to amplify the signal centered at 1550 nm. The secondary pump presents a nearly constant combined forward- and backward-propagating power, and accordingly can provide a nearly constant gain for the optical signal at 1550 nm. The gain can be adjusted to closely match the signal attenuation at every step of the propagation. An experimental verification of this concept has been reported by Ania-Castañón et al. in an article “Ultralong Raman Fiber Lasers as Virtually Lossless Optical Media”, Phys. Rev. Lett. 2006 Vol. 96, 023902, reporting a lossless (+−0.2 dB) 70 km long transmission link. Detrimentally, the lasing cavity can create noise caused by optical instability of lasing in a multi-kilometer long optical cavity.
Stentz et al. in U.S. Pat. No. 6,163,636 disclose an optical communication system including multiple-order Raman amplifiers. Raman pumps of second order are used to amplify Raman pump light of the first order, which then amplifies optical signal. This allows the optical signal power distribution to become more even, reducing ASE noise or improving Noise Figure.
Papernyj et al. in U.S. Pat. No. 6,480,326 disclose an optical fiber communication system similar to that of Stentz et al. The optical fiber communication system of Papernyj et al. includes “seed” Raman pumps. Referring to FIG. 1A, a transmission system 100 includes a transmission fiber 5 between first 1 and second 2 terminals, a primary Raman pump 6 coupled to the first terminal 1 via wavelength division multiplexors (WDMs) 7 and 11 coupled by a fiber link 12. Seed sources 8 and 9 are coupled to the first terminal 1 via WDMs 10 and 11. In operation, the primary Raman pump 6 pumps the transmission fiber 5. The primary pump light interacts with light provided by the seed sources 8 and 9, causing the seed light to gain power as it propagates through the transmission fiber 5. As the seed light gains power, it begins to pump the optical signal propagating from the first terminal 1 to the second terminal 2, thus lessening the signal power variation. For instance, referring to FIG. 1B, the signal power variation is lessened from 14 dB peak-to-peak in a previous prior art system (solid line) to approximately 8 dB peak-to-peak in the transmission system 100 (dashed line).
The prior art, while providing means for lessening optical signal power variation along an optical fiber span, does not yet provide a capability to reliably achieve a substantially “lossless” transmission, where optical signal power is stable in time and varies insignificantly over long optical fiber spans.